Jet-Printed Solution-Processed MoS₂ Semiconductor and Jet-Printed Al Electrodes for Thin-Film Transistors

Recently, transition-metal dichalcogenides (TMDs) have attracted much attention as new materials for electronics devices, electrocatalysts, photocatalysts, sensors, batteries, and bio-applications. Most TMDs are two-dimensional (2D) materials with a single layer. Bonds between each layer are made up of Van der Waals bonds, while intra-layer atoms bind together as covalent bonds.
Chemical vapor deposition (CVD) with sulfur gas is the most popular method for synthesizing large- scale 2D materials with high quality. Various MoS₂ can be obtained from this method using various precursors with different properties, process temperatures, and substrate materials. Solution process methods show advantages for preparing films with large size, high throughput, low cost, thickness control, and an environmentally friendly process. Even though there is sulfur in the precursors of the solution-process synthesis methods, supplementing the sulfur that is lost in the high-temperature CVD process is unavoidable.
Electrohydrodynamic (EHD) jet printing is a technique that uses electric fields to yield fluid flows for delivering solution/paste materials to a target substrate. EHD printing can create smooth areas or patterns with a large range of material viscosity, even with low-viscosity solutions or high-viscosity pastes, which are better by far than inkjet printing for this concern. Moreover, the merit of EHD jet printing is that MoS₂ TFTs can be patterned simply without using any shadow masks compared to other methods such as E-beam or thermal evaporation.
Directly printed transistors have been in the limelight due to low cost and an environmentally friendly technique. An electrohydrodynamic (EHD) jet printing technique was employed to pattern both MoS₂ active layer and Ag source/drain electrodes. Printed MoS₂ lines were patterned on a silicon wafer using a precursor solution of ammonium tetrathiomolybdate and ammonium molybdate in different solvent formulations. Simple thermal annealing with bottom-up thermolysis methods were used to make MoS₂ multi-crystalline, without CVD. These patterns were transferred on other SiO₂ substrates as semiconductors for TFT fabrications. On top of the patterned MoS₂, Ag paste was also patterned for source and drain electrodes using EHD jet printing, too. The electrical properties show improved mobilities of 9 to 48 cm²/V s and reasonable on-off ratios of around 1.0×10⁵ with solid output saturations and better hysteresis behaviors. This result could be important for practical TFT applications and could be extended to other 2D materials.